Sputter target manufacturers have relied upon “as cast” as a low-cost method for producing precious metal sputter targets. In addition to this, manufacturers have relied-upon conventional metalworking and annealing to produce precious metal sputter targets having a more uniform grain structure. Unfortunately, these techniques provide limited benefit for sputter target manufacturers. For example, typical rolled and annealed platinum group sputter targets have a grain size of about 300 to 5000 μm.
Target manufacturers have relied upon equal channel angular extrusion (ECAE) to produce fine grain microstructures. Nakashima et al., “Influence of Channel Angle on the Development of Ultrafine Grains in Equal-Channel Angular Pressing,” Acta. Mater., Vol. 46, (1998), pp. 1589-1599 and R. Z. Valiev et al., “Structure and Mechanical Behavior of Ultrafine-Grained Metals and Alloys Subjected to Intense Plastic Deformation,” Phys. Metal. Metallog., Vol. 85, (1998), pp. 367-377 provide examples of using ECAE to reduce grain size. ECAE introduces an enormous strain into a metal without imparting significant changes in workpiece shape. In fact sputter target manufacturers have claimed an ability to use ECAE to reduce the grain size of high-purity copper sputter targets to less than 5 μm. Although this process is effective for reducing grain size, it does not appear to align grains in a manner that facilitates uniform sputtering or provide an acceptable yield—the low yield originates from the ECAE process operating only with rectangular shaped plate; and thus, requiring an inefficient step of cutting circular targets from the rectangular plate.
Zhu, et al., in U.S. Pat. No. 6,197,129 B1, entitled “Method for Producing Ultrafine-Grained Materials using Repetitive Corrugation and Straightening” disclose a method for reducing grain size by intense redundant strains. Unfortunately, the repeated reversing bending strains would likely produce severe strains at the workpiece surfaces, while imparting only limited strains to the workpiece's mid-thickness regions, causing a gradient in material microstructure from surface to mid-thickness. These material property gradients are not suitable for sputter targets, as through-thickness property uniformity is a critical factor for consistent performance of a sputter target.
Lo, et al., in U.S. Pat. No. 5,766,380, entitled “Method for Fabricating Randomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains and Fine Precipitates” disclose a cryogenic method for fabricating aluminum alloy sputter targets. This method uses cryogenic processing with a final annealing step to recrystallize the grains and control grain structure. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621, uses cryogenic working and annealing to manipulate and enhance crystallographic texture of titanium sputter targets.